11:45 〜 12:00
[SCG46-05] 鉱物炭酸塩化および含水化における反応誘起破壊のスケーリング則
キーワード：鉱物炭酸塩化、蛇紋岩化、反応誘起破壊、ブルース石
Mineral carbonation and hydration in the lithosphere are one of the key reactions that govern the carbon and water cycles in the Earth as well as artificial fixation of carbon dioxides[1]. These reactions generally involve a large volume increase of ~tens to a hundred percent, which may clog the pores in the rock and hinder further reactions. On the other hand, observations of natural carbonation and hydration reactions suggest that rocks may fracture by volume-increase of the reaction (i.e., reaction-induced fracturing), and thereby are enhanced for fluid flow and further reactions[1]. However, up to now, most of the laboratory experiments on mineral carbonation have not reproduce these processes, and results in clogging and/or decrease in permeability. As such, the factors controlling the mechanical and hydraulic responses in volume-increasing reactions remain largely unknown.
Here we review our recent experiments that produces reaction-induced fracturing[2][3], and combined with the compilation of other experimental and natural reaction textures, we propose a scaling-law for reaction-induced fracturing during mineral carbonation and hydration.
During reactions of brucite-rich serpentinite with NaHCO3 solution, we found that brucite preferentially react with CO2 to produce magnesite within the rock, and produce macroscopic fracturing, which further enhanced carbonation. On the other hand, reactions of the same rock with CO2-saturated water produced magnesite on the sample surface, and did not form fractures[3]. Geochemical modeling of brucite carbonation show that Mg concentration in the solution is relatively lower than that of dissolved CO2 in NaHCO3 solution, whereas Mg concentration is higher than CO2 concentration in CO2-saturated water, suggesting that the rate of Mg transport to the surface relative to the rate of CO2 transport into the sample interior controls the location of magnesite precipitation, thereby control the reaction-induced fracturing.
Observations of reaction textures in the natural system such as serpentinization and olivine carbonation suggest that reaction products precipitate within the rocks and cause fracturing. On the other hand, observation of most of the laboratory experiments on serpentinization and carbonation show that reaction products may not directly precipitate within the rocks but preferentially precipitate on the surface of rocks or spaces in pre-existing fractures. Such contrasting behavior of precipitation locations between natural system and experimental system can be explained by the difference in the size of system, thereby transport distance to the free surface: natural system has a long transport distance to macroscopic fractures or block surfaces (i.e., ~10s m), therefore reactants precipitate within the rock. On the other hand, in the experimental system, the transport distance is as short as 100s µm to centimeters, depending on the size of the samples, and it is much easier to transport dissolved cations to the surface of the sample, and precipitate on the surface.
Such effects of transport rate and system size on the mechanical behavior is further supported by the observation that extremely fast hydration reaction of periclase causes macroscopic fracturing in laboratory[2] and that serpentine precipitate on the surface of partly serpentinized peridotite blocks when block size is small (<decimeter) in natural system[4].
Based on these discussions, we propose that the competition of the rate of cation transport to the rock surface and the rate of CO2 or H2O supply to the rock interior control the location of the mineral precipitation, and control the mechanical and hydraulic behavior of the volume-increasing fluid-rock reactions.
[1]: Kelemen and Matter 2008 PNAS, 105, 17295–17300.
[2]: Uno et al. 2022 PNAS, 119, e2110776118.
[3]: Igarashi et al. 2023 JpGU abstract.
[4]: Uno and Kirby 2019 Lithos, 336–337, 276–292.
Here we review our recent experiments that produces reaction-induced fracturing[2][3], and combined with the compilation of other experimental and natural reaction textures, we propose a scaling-law for reaction-induced fracturing during mineral carbonation and hydration.
During reactions of brucite-rich serpentinite with NaHCO3 solution, we found that brucite preferentially react with CO2 to produce magnesite within the rock, and produce macroscopic fracturing, which further enhanced carbonation. On the other hand, reactions of the same rock with CO2-saturated water produced magnesite on the sample surface, and did not form fractures[3]. Geochemical modeling of brucite carbonation show that Mg concentration in the solution is relatively lower than that of dissolved CO2 in NaHCO3 solution, whereas Mg concentration is higher than CO2 concentration in CO2-saturated water, suggesting that the rate of Mg transport to the surface relative to the rate of CO2 transport into the sample interior controls the location of magnesite precipitation, thereby control the reaction-induced fracturing.
Observations of reaction textures in the natural system such as serpentinization and olivine carbonation suggest that reaction products precipitate within the rocks and cause fracturing. On the other hand, observation of most of the laboratory experiments on serpentinization and carbonation show that reaction products may not directly precipitate within the rocks but preferentially precipitate on the surface of rocks or spaces in pre-existing fractures. Such contrasting behavior of precipitation locations between natural system and experimental system can be explained by the difference in the size of system, thereby transport distance to the free surface: natural system has a long transport distance to macroscopic fractures or block surfaces (i.e., ~10s m), therefore reactants precipitate within the rock. On the other hand, in the experimental system, the transport distance is as short as 100s µm to centimeters, depending on the size of the samples, and it is much easier to transport dissolved cations to the surface of the sample, and precipitate on the surface.
Such effects of transport rate and system size on the mechanical behavior is further supported by the observation that extremely fast hydration reaction of periclase causes macroscopic fracturing in laboratory[2] and that serpentine precipitate on the surface of partly serpentinized peridotite blocks when block size is small (<decimeter) in natural system[4].
Based on these discussions, we propose that the competition of the rate of cation transport to the rock surface and the rate of CO2 or H2O supply to the rock interior control the location of the mineral precipitation, and control the mechanical and hydraulic behavior of the volume-increasing fluid-rock reactions.
[1]: Kelemen and Matter 2008 PNAS, 105, 17295–17300.
[2]: Uno et al. 2022 PNAS, 119, e2110776118.
[3]: Igarashi et al. 2023 JpGU abstract.
[4]: Uno and Kirby 2019 Lithos, 336–337, 276–292.